The Background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present disclosure.
Communications systems such as cellular systems and wireless systems allow users to transmit and receive data wirelessly between users and/or between users and a cell station. Typically, the cellular and wireless systems must operate at a specific frequency and a below a specific power level. Within those constraints, the cellular and wireless systems attempt to maximize data transfer for individual users while accommodating the demands of all of the other users that share the cellular or wireless system. Therefore, each wireless device must wisely use allocated bandwidth to maximize data transfer. Designers of these systems may also be limited by market demands for low cost devices and ongoing cost of operation.
There are a number of approaches that have been developed to maximize the use of the allocated bandwidth while minimizing interference between cellular and wireless users. For example, one approach involves allocating the available bandwidth using time division multiple access (TDMA). TDMA is a digital signal transmission scheme that allows multiple users to access a single radio-frequency (RF) channel. Interference between channels is avoided by allocating unique time slots to each user within each channel. Other approaches include spread spectrum techniques that involve spreading or splitting transmit signals over multiple different frequencies and recombining the signal at a receiver. Spread spectrum approaches typically tend to be more complex and increase the cost of the wireless device and the overall cost of operation.
Various different types of communications systems employ TDMA. For example, cellular systems often use TDMA. Once cellular system that uses TDMA is a Personal Handy-phone System (PHS), which is a mobile telephone system that operates in the 1.88-1.93 GHz frequency band. PHS has been popular in markets with strong demand for low cost cellular phones and cost of operation, PHS is a cordless telephone system with capability to handover signals from one cell to another. PHS cells are smaller than cells of cellular phone systems that use Global System for Mobile communication (GSM).
Typically, PHS has a transmission power of 500 mW and a range of 10-100 meters. PHS provides service with minimal congestion in areas of heavy call-traffic such as business districts, downtown, etc. This is accomplished by installing cell stations at a radial distance of every 100-200 meters. Thus, PHS is particularly suitable for use in urban areas.
PHS-based phones can be used in homes, offices, and outdoors. PHS offers a cost-effective alternative to conventional phone systems that use ground lines. Additionally, PHS-based phones can interface with conventional phone systems. Thus, where ground lines of conventional phone systems cannot reach a physical location of a subscriber, the subscriber can use PHS to reach the conventional phone system and establish communication with other subscribers served by the conventional phone system.
PHS uses time division multiple access (TDMA) as radio interface and adaptive differential pulse code modulation (ADPCM) as voice coder-decoder (codec). A codec includes an analog-to-digital converter (ADC) and a digital-to-analog converter (DAC) that translate signals between analog and digital formats. TDMA is a digital signal transmission scheme that allows multiple users to access a single radio-frequency (RF) channel. Interference between channels is avoided by allocating unique time slots to each user within each channel. For example, a PHS frame comprises four channels: one control channel and three traffic channels.
Unlike PCM codecs that quantize speech signals directly, ADPCM codecs quantize a difference between a speech signal and a prediction made of the speech signal. If the prediction is accurate, the difference between actual and predicted speech may have a lower variance than variance in actual speech. Additionally, the difference may be accurately quantized with fewer bits than the number of bits that would be needed to quantize the actual speech. While decoding, a quantized difference signal is added to a predicted signal to reconstruct an original speech signal. The performance of the codec is aided by using adaptive prediction and quantization so that a predictor and a difference quantizer adapt to changing characteristics of speech being coded.
Referring now to FIG. 1, a PHS phone system includes a PHS phone 10 with an antenna 12 and a cell station 11 having an antenna 13. An exemplary PHS phone 10 comprises a signal processing module 16 including a transmit module 18 and a receive module 20, memory 22, a power supply 24, and an I/O module 26. The I/O module 26 may comprise various user-interfaces such as a microphone 26-1, a speaker 26-2, a display 26-3, a keypad 26-4, a camera 26-5, etc.
The transmit module 18 converts user input from the microphone 26-1 into PHS-compatible signals. The receive module 20 converts data received from the antenna 12 into a user-recognizable format and outputs the same via speaker 26-2. The signal processing module 16 uses memory 22 to process data transmitted to and received from the antenna 12. The power supply 24 provides power to the phone 10.
Digital data is typically represented by zeros and ones, which are called bits. Data is generally transmitted by modulating amplitude, frequency, or phase of a carrier signal with a base-band information-bearing signal. Quadrature phase shift keying (QPSK) is a form of phase modulation generally used in communication systems. In QPSK, information bits are grouped in pairs called dibits. Thus, QPSK uses four symbols that represent dibit values 00, 01, 10, and 11. QPSK maps the four symbols to four fixed phase angles. For example, symbol 00 may be mapped to (+3π/4). On the other hand, π/4-DQPSK uses differential encoding wherein mapping between symbols and phase angle varies. Additionally, π/4-DQPSK maps each of the four symbols to a real and an imaginary phase angle resulting in an eight-point constellation.
Referring now to FIGS. 2A-2B, the transmit module 18 comprises an ADPCM module 50, a framer module 52, a serial-to-parallel converter module 54, a DQPSK mapper module 56, a square-root raised cosine (SRRC) filter module 58, and an upsample module 60. The receive module 20 comprises a downsample module 70, an automatic gain control (AGC) module 72, a demodulator 75 including a carrier acquisition module 74 and an equalization module 76, a de-mapper and parallel-to-serial converter module 78, a de-framer module 80, and an ADPCM module 82.
When transmitting data from the phone 10 on a channel, the ADPCM module 50 converts audio and/or video signal into bits of digital data. The framer module 52 partitions the digital data into frames. The serial-to-parallel converter module 54 converts the bits in the frames into symbols. The DQPSK mapper module 56, which may utilize a modulation scheme such as π/4-DQPSK modulation, maps four real and four imaginary values of four symbols in each frame to a total of eight phase angles and generates a complex baseband signal.
The SRRC filter module 58, which is essentially a Nyquist pulse-shaping filter, limits the bandwidth of the signal. Additionally, the SRRC filter module 58 removes mixer products from the complex baseband signal. The upsample module 60 comprises a quadrature carrier oscillator that is used to convert the phase-modulated baseband signal into a phase-modulated carrier signal. The upsample module 60 transmits the phase-modulated carrier signal on the channel at a sampling frequency that is greater than twice the Nyquist frequency.
When the phone 10 receives a signal from the antenna 12, the downsample module 70 downsamples the signal using an asynchronous oscillator. The downsample module 70 down-converts the signal from the phase-modulated carrier signal to the phase modulated baseband signal. The AGC module 72 maintains the gain of the signal relatively constant despite variation in input signal strength due to transmission losses, noise, interference, etc.
The carrier acquisition module 74 demodulates the signal, retrieves carrier phase information, and decodes symbol values from the signal. The equalization module 76 corrects any distortion present in the signal. The de-mapper and parallel-to-serial converter module 78 de-maps and converts the demodulated signal into a serial bit-stream. The de-framer module 80 de-partitions the frames into digital data bits. The ADPCM module 82 converts the digital data bits into audio and/or video data and outputs the data to the speaker 26-2 and/or the display 26-3 of the phone 10.
Legacy communications systems such as the Personal Handy-phone System (PHS) are configured to be simple and low cost. Differential demodulation has been adopted based on the technological constraints present at the time of designing these systems for baseband demodulation and therefore does not have the capability to combat inter-symbol-interference (ISI) typically introduced by multi-path fading.